In the field of audiology, it is necessary to deliver a predetermined sound level to a specific point. This point can be the pinna of the ear or the transducer surface of a hearing aid. Heretofore, the measurement of either a passive device such as the hearing aid or the response of the human ear has been accomplished by first calibrating a sound source to a predetermined point in a "sound booth" and then transmitting calibrated levels of sound to the unit under test or patient's ear. However, once the individual or the device is placed into the sound booth, the acoustics of that particular enclosure may change, thus changing the calibration.
To minimize calibration errors when testing a patient's hearing, headphones are normally used with the assumption that the distance from the headphone transducer to the external auditory canal is a constant. Once the headphones are in place, the patient can then provide an evoked response to various tone patterns that are supplied to the individual. However, as described above, the test system for obtaining the evoked response normally requires a set of calibrated headphones and a sound booth. When testing a patient's hearing in a "free field", calibration and control of sound pressure levels at the ear becomes more difficult. This is due to the fact that sound pressure varies as a function of the distance between the patient and the sound source.
In calibrating a system for either measuring a device such as a hearing aid or calibrating the sound pressure level (SPL) at a remote point such as the human ear, it is necessary to have both a precision transducer at the remote point and a knowledge of the loss and frequency response of the medium through which the sound wave is being transmitted. In addition, it is also necessary to minimize reflections from foreign objects proximate the remote point that may result in reflections which can set up interference patterns proximate the remote point. These interference patterns can severely detract from the accuracy of the measurement performed.
In order to simplify measurements at a given point, a measurement technique has been developed which produces a time delay spectrum. This is referred to as Time Delay Spectrometry (TDS) which is disclosed in U.S. Pat. No. 4,279,019. This technique utilizes a frequency generator disposed at one point termed the "source" and a sound transducer disposed at a remote test point. The frequency is generated for a very short duration of time and this frequency is sampled at a predetermined interval of time which is adjustable. The interval of time is equal to the delay which occurs as the sound passes through the transmission medium such as air. The sample is only taken for a short interval of time to prevent reflections from other boundaries proximate the remote test point from being summed with the sample taken. In this manner, it is possible to measure only the SPL that results from the transmitted frequency that is transmitted through the medium to the transducer at the remote point. Since propagation of sound through the medium undergoes some form of spherical spreading, the energy actually received at the remote test point is substantially attenuated with the remaining energy available for reflection off of different boundaries.
The above described TDS technique utilizes a swept frequency response and a narrow bandpass filter wherein the actual sweeping of a frequency source prevents any given frequency from being present for a long duration of time. The results provided by the TDS technique are a plot of SPL versus frequency. However, no information is provided regarding the reflections and the effect that they may have upon an evoked response test. This problem is addressed by another technique that has resulted from the original TDS technology and is disclosed in U.S. Pat. No. 4,279,019 issued to the same inventor. This technique provides an energy time curve for a given swept frequency response. In this technique, a "chirp" signal is output from the source and the Fast Fourier Transform of the received signal is taken. This provides the energy time curve (ETC) of the source signal. This ETC curve provides information regarding such things as reflections from other boundaries, etc.
Although the TDS technique and the ETC technique have been developed to aid in testing of devices in the various mediums without the reflections from objects within the medium interferring, the techniques at present are inadequate for application to the audiology industry since they are still time consuming at best. Therefore, there exists a need for a system which applies the concepts founds in TED and ETC to enhance testing of both the hearing response of an individual and the frequency response of a device such as a hearing aid.